Thermal runaway during overcharge in the lithium ion batteries (LIB) is a safety concern in the automotive industry. Layered metal oxide cathode materials, in particular, impose higher risks of thermal runaway due to their exothermic oxidative reactions with the electrolyte. Battery chargers usually operate at a fixed current or at a fixed power. Consider the case of a fixed current applied to a lithium ion battery containing a lithium metal oxide. The total current leaving each electrode is the sum of all the electrochemical reactions occurring in that electrode. The reactions that may be occurring at the positive electrode during overcharge include: (1) extraction of lithium ions from the lithium metal oxide, (2) side reactions which generate only inert gaseous species, and (3) side reactions which generate gas and/or species which may continue to react after interruption of the current. The relative rates of the reactions depend on the difference between the local electrochemical potential and the redox potential for the particular reaction, which is a function of the local reactant and product concentrations. For the case of a lithium metal oxide, once all of the lithium has been extracted at the redox potential of the oxide, no further oxidation of the active material is possible. Continued application of a charging current will then drive up the cell voltage to potentials at which the side reactions occur at rates sufficient to meet the applied current. That high voltage may lead to unwanted side reactions which generate species that continue to react after the current is interrupted or which generate gas at too rapid a rate, potentially leading to thermal runaway.
Traditionally, electrolyte additives have been employed for the overcharge protection of commercial-grade lithium ion batteries. Examples of such additives are redox shuttles which bypass the overcharge current via a redox reaction between the cathode and the anode. Such compounds usually have redox potentials of 0.2-0.4 V higher than the end-of-charge potential of the positive electrode, or cathode. In another approach, polymer precursors, for instance aromatic compounds such as cyclohexyl benzene and biphenyl are electrochemically oxidized and polymerized thereafter on an overcharged positive electrode to form a passive film that prevents the electrolyte from further reacting with the positive electrode.